The present invention relates to sensors for monitoring the concentration of electro-active materials and, more particularly, it relates to a methanol concentration sensing apparatus whose construction is simple and whose response is fast, accurate, reproducible, and reliable.
Sensing methanol concentration is not a trivial task. A few sensing mechanisms have been explored to date. In U.S. Pat. No. 4,810,597, granted to Kumagai et al on Mar. 7, 1989, the open circuit voltage change caused by the presence of methanol was used as a means to determine methanol concentration. The structure of the sensing device looked like a regular direct methanol fuel cell (DMFC). When methanol diffused through the membrane to the air cathode side, the voltage of the cathode declined. A higher methanol concentration caused more methanol to diffuse through, resulting in a lower cathode open circuit voltage. Applicants have tried to use this method to determine methanol concentration, but found the reproducibility of this technique was too poor.
Capacitance was also used as a means of monitoring methanol concentration in a mixture of gasoline and methanol, as illustrated in U.S. Pat. No. 4,939,467, granted to Nogami et al on Jul. 3, 1990, and U.S. Pat. No. 5,196,801, granted to Nogami et al on Mar. 23, 1993. Due to the difference in dielectric constants between methanol and gasoline, the capacitance between two electrodes changes with the methanol concentration. Unfortunately, since the dielectric constants of water and methanol are very close and the methanol concentration used in a DMFC is normally less than 5% wt., such a method cannot provide a reliable measure of methanol concentration in water.
In recent times, there has been appreciable effort to develop and commercialize direct methanol fuel cells. These methanol fuel cells will require a sensor to continuously monitor the concentration of the methanol solution fed to the fuel cell anode.
The electrochemical oxidation of methanol has been used by two groups of scientists to measure methanol concentration for DMFCs. FIG. 1c shows a design based on measuring the limiting current output from the oxidation of the methanol that diffuses through a Nafion 117 proton-exchange membrane, as illustrated by Barton et al in J. Electrochem. Soc., vol. 11, pp. 3783-3788, 1998, November. Methanol in the fuel diffuses through the membrane to the cathode side of the cell where it is oxidized to carbon dioxide and protons by an applied voltage, as shown by FIG. 1d. The reaction on the methanol side of the fuel cell comprises the reduction of protons to hydrogen. The protons come from the methanol oxidation occurring on the other side of the membrane, which then back diffuse through the membrane. The applied voltage is high enough so the transport of methanol through the membrane is the limiting factor in determining the current. Higher methanol concentration in the fuel will result in a higher limiting current. They demonstrated a strong correlation between the limiting current and methanol concentration up to a methanol concentration of 4 M over a temperature range of 40 to 80xc2x0 C. They also stated that the transient current response to methanol concentration change indicated a response time of about 10 to 50 seconds. The variation depends primarily on temperature.
Following the same electrochemical principle, Narayanan et al illustrated a similar design in Electrochemical and Solid-State Letters, vol. 3, pp. 117-120, 2000, March, as shown in FIG. 1e. It was also illustrated in WO 98/45694, granted to Narayanan et al on Oct. 15, 1998, and U.S. Pat. No. 6,306,285, granted to Narayanan et al on Oct. 23, 2001. The major difference from Barton""s design was that the methanol was oxidized by an applied voltage at the methanol side as shown in FIG. 1f. The entire sensing cell could be immersed in the methanol solution with both its anode and cathode being in contact with methanol solution. It is the polarity of the applied voltage that determines which side is anode and which side is cathode. The inventors reported a response time of less than 1 second and a nonlinear correlation between the oxidation current and methanol concentration up toga methanol concentration of ca. 2.0 M.
The sensing apparatus in both Barton""s design and Narayanan""s design was actually a small DMFC single cell. The reactions occurring on the electrodes are depicted below:
Positive side: CH3OH+H2Oxe2x86x92CO2+6H++6exe2x88x92xe2x80x83xe2x80x83(1)
Negative side: 6H++6exe2x88x92xe2x86x923H2xe2x80x83xe2x80x83(2)
Since both Barton et al and Narayanan et al used a flat DMFC single cell as the sensing apparatus, using these xe2x80x9csensorsxe2x80x9d in a real DMFC system is problematical. In Barton""s design it is critical to have a good seal among all the sensing components so that methanol will arrive at the positive side only via diffusion. In Narayanan""s design it is difficult to predict how the methanol solution flows over the sensing electrode surface. Furthermore, it seems to be quite difficult to incorporate such a xe2x80x9csensorxe2x80x9d into a DMFC system.
The present invention comprises a methanol concentration sensing apparatus using the same electrochemical principle as Barton and Narayanan cells, but it is dramatically simpler to construct and use. The sensor is highly reliable and sensitive with a fast response time.
The sensing device comprises a flexible composite of layered materials wrapped about a flexible tube having aperture contact with a methanol flow stream. Since all the materials used are flexible and bendable, there is no problem fastening the assembly along the curled outer surface of a piece of tubing using clamps or other types of fasteners. Because of the simplicity of its design such a sensor can be fabricated in minutes. This tubing, with the fastened assembly, is easily connected to the inlet of the methanol solution flow loop for a DMFC. The sensor can actually be connected anywhere in the methanol solution flow loop. A preferred location is within the methanol solution mixing tank.
In contrast, both Barton""s and Narayanan""s designs have inflexible sensors, making it impossible to fasten them on a curved surface. Hence, their sensors cannot be easily adapted into the methanol solution flow loop. In addition, the engineering needed to fabricate these related-art sensing cells is much more complex.
In accordance with the present invention, a sensing device is featured that electrochemically measures methanol concentration. The sensing device comprises a flexible composite of layered materials wrapped about a flexible tube having aperture contact with a methanol flow stream. The layered materials sequentially wrapped on the tube comprise: a polytetrafluoroethylene insulation sheet; an electrically conducting mesh representing the anode current collector; a carbon-based material representing an anode diffusion medium; a catalyst-coated membrane with both sides coated by catalysts such as Pt/Ru and Pt; a carbon-based material serving as the cathode diffusion medium; and an electrically conducting mesh representing the cathode current collector.
It is one object of the present invention to provide an improved, inexpensive, and easy-to-use methanol sensor.
It is another object of this invention to provide a methanol sensor that is in direct contact with the methanol solution flow stream for the fuel cell.
It is still another object of this invention to provide a sensor for monitoring the concentration of electro-active materials.